Excursion Report
A small group of members examined the Carrock Fell Complex spending most of their time in the southern mafic plutonic component considered to be associated with the Eycott Volcanic Group. Â Discussion covered the range of published interpretations for this body of layered cumulate gabbros pooling our collective knowledge of magma-chamber processes and guided by Prof. Joe Cann. Â The gabbroic body is bounded by sub-vertical faults and the internal igneous layering is steeply inclined. Â Published explanations cover a spectrum from layered gabbro accumulation in a sub-horizontal sheet followed by high degrees of rotation to crystallisation on an inclined floor of a magma chamber with no subsequent rotation. Â If the former proposal is correct, the challenge is to understand how and when the very significant rotation happened; could it have been in the murky events associated with the initiation of subduction at the leading edge of Avalonia or a volcano-tectonic fault, or a younger event? Â Lighter and darker banding in the gabbro was first seen in the quarry along the road from Mosedale Bridge. Â Within the mafic body we observed the layering mainly dipping at around 50 degrees though steeper dips are found on the margin. Â The party climbed up the steep slopes of Carrock Fell, approximately along the steep contact of the gabbro with the metamorphosed and deformed Skiddaw Group. Â Some large xenoliths of intensely folded hornfelsed Skiddaw rock were seen within the intrusion in amongst moderate-sized exposures of layered gabbro.
On the summit of Carrock Fell, the group encountered the second, and younger, component of the complex which is a gabbro to microgranite intrusive suite unrelated to the gabbro cumulates. Â Here exposed granophyre weathers to a distinctive pinkish colour. Â Boulders of gabbro on the summit occur as sparse erratics, carried upwards and northwards by ice.
All agreed that the Carrock Fell complex is indeed complex, and would merit further study.
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